The high pressure sodium discharge lamp with its highly efficient golden-yellow discharge has made a tremendous impact on city street and highway lighting. One of the most critical operations in the manufacture of a high pressure sodium discharge lamp is the sealing of the refractory metal end caps to the polycrystalline alumina or sapphire arc tube body. Additionally, most early lamp failures can be traced to a breakdown of the seal between the arc tube and its end caps, and can in many cases be further identified as a failure of the bond between the sealing frit and the end cap at their interface.
Commercial high pressure sodium discharge lamps employ a glassy sealing frit to bond the arc tube body to the refractory metal end caps, and that glassy sealing frit in almost all instances principally comprises aluminum oxide and calcium oxide in about eutectic proportions. Most of these sealing frits generally include small quantities of other metallic oxides such as silicon dioxide, magnesium oxide, barium oxide, yttrium oxide, etc. Several of these sealing compositions along with the method by which the polycrystalline alumina arc tube is bonded to the refractory metal end cap in the high pressure sodium discharge lamp are disclosed in U.S. Pat. No. 3,281,309 to J. F. Ross; U.S. Pat. No. 3,469,729 to R. B. Grekila et al; and U.S. Pat. No. 3,588,577 to Charles I. McVey et al. The inadequacies of the bond between the sealing glass frit and the refractory metal end cap has been previously recognized and efforts have been continuing to solve this problem. One attempt at a solution is disclosed in U.S. Pat. No. 3,448,319 to W. C. Louden in which a suspension of tungsten trioxide in a suitable binder mixed with a minor proportion of the sealing composition was coated on the interior surface of the end cap. In that process, great care had to be taken to assure that the tungsten layer was completely overcoated with a layer of ceramic sealing material so that none of the tungsten would be exposed to the alkaline metal vapor in the arc tube. U.S. Pat. No. 3,598,435 to Paul J. Jorgensen discloses a process wherein zirconium dioxide is formed on the niobium by coating the refractory metal with zirconium hydride or alternatively employing zirconium oxide or a zirconium rich niobium alloy by diffusion of zirconium into the niobium surface. The use of zirconium however, is believed to cause undesirable embrittlement of the niobium end cap.